Missing the target: Characterization of bullous pemphigoid patients who are negative using the BP180 enzyme-linked immunosorbant assay

Background: Bullous pemphigoid (BP) is an autoimmune blistering disease characterized by autoantibodies specific for the 180-kd BP antigen-2 (BP180) (also termed "type XVII collagen'') protein. The BP180 enzyme-linked immunosorbent assay (ELISA) is specific for the immunodominant NC16A domain of the protein. However, we and others have observed patients whose reactivity to BP180 is exclusive of the NC16A domain (referred to henceforth as non-NC16A BP). Objective: We sought to determine the incidence of non-NC16A BP and identify regions of reactivity within the BP180 protein. Methods: Sera from 51 patients who met the clinical and histologic criteria for BP were screened for NC16A reactivity by ELISA. Sera that were negative by ELISA were screened for IgG reactivity to an epidermal extract, recombinant BP180 protein, and subregions of BP180, by immunoblot. Demographic and clinical data were also collected on all patients. Results: Four sera (7.8%) were negative using the BP180 ELISA but positive for IgG reactivity to the extracellular domain of BP180. Further mapping identified 4 regions outside of NC16A recognized by these sera: amino acid (AA) 1280 to 1315, AA 1080 to 1107, AA 1331 to 1404, and AA 1365 to 1413. One of these sera also had IgE specific for NC16A. One patient had an atypical presentation with lesions limited to the lower aspect of the legs and scarring of the nail beds. Limitations: The small total number of patients with non-NC16A BP limits the identification of demographic or clinical correlates. Conclusion: It is significant that 7.8% of sera from patients with new BP react to regions of BP180 exclusively outside of NC16A and, thus, would not be identified using the currently available BP180 ELISA. (J Am Acad Dermatol 2013;68:395-403.)